You can put them pretty much anywhere. The surface.GetTexureID starts in the .luacraft directory.
If you wanted the default terrain.png as a texture, you can just do surface.GetTexureID( "/terrain.png" )

Please re-download the ESP addon. I fixed a small bug regarding IsAlive() and IsLiving() yesterday!
Also addons always worked, I just made it very unclear if they were enabled or not, sorry for the confusion :D

It got disabled because it was bugged I suppose. Also, if you want to code in-game press the ` or ~ key, depends on your keyboard.. But press that key, and then type lua "Code Here" No Quotations... Cheers.

Is there some way to include other lua files? For example, I have a lua file named arena.lua in the same folder as my init.lua. Inside arena.lua I stuck module(..., package.seeall) at the top, but on requiring 'arena', I get a lua error telling me that arena.lua does not exist, even though I can obviously see it right there. Is there some sort of directory structure or include directory that I should use?